OUI (Organizationally Unique Identifier) is the first 24 bits (first three octets) of a MAC address. It identifies the manufacturer of the network interface. The IEEE Registration Authority assigns each hardware vendor one or more OUIs.
Format
For MAC d4:81:d7:2c:5f:8a:
- OUI =
d4:81:d7(Cisco Systems) - Device serial =
2c:5f:8a
OUI Assignments (Examples)
| OUI | Vendor |
|---|---|
00:1D:0F | Apple |
AC:DE:48 | Cisco Meraki |
F4:5C:89 | Google (Nest) |
B8:27:EB | Raspberry Pi |
52:54:00 | QEMU virtual |
00:0C:29 | VMware virtual |
OUI Size Variants
- OUI-24 (24-bit) — historical, largest allocation
- OUI-28 (28-bit) — smaller allocation for vendors with fewer devices
- OUI-36 (36-bit) — even smaller
Modern IEEE registrations use OUI-24 for large vendors and OUI-28/36 for smaller ones.
Privacy: Randomized MAC
When a modern device uses a randomized MAC (iOS 14+, Android 10+), the OUI portion is set to a “locally administered” value — the second-least-significant bit of the first byte is 1. This tells other devices: “this MAC isn’t tied to a specific vendor.”
Lookup
DomainScan’s MAC address lookup tool extracts the OUI from any MAC and returns the assigned vendor name.
Related
Check the MAC Address glossary entry, read the What Is a MAC Address? deep dive, and see the Port 25 entry for another key network identifier.